According the the "open letter", the Onyx Panther puppy will be ONLY available in the "Limited Time Edition" and no-where else. A similar thing is going to be there for the rest of the three announced puppies :

but remember: if you love the Demonweb Spiderling, the Druid’s Wolf Pup, and the Onyx Panther Cub make sure that you pick up the expansion before June 24th, because these particular Companions are exclusive and won’t be in the store once pre-purchase ends!

Too bad that I would only be interested in the panther.
Personally, I'm not interested at all in a mini spider. It's not that I don't like spiders (I don't like them that much in fact, although I find spiders to be interesting and fascinating animals), it is only so that a spider just doesn't "tune in" with my personality - or those of my game characters at all.

so we have arranged for all existing characters in DDO to get their very own slice of Six Year Old Cake. While that may not sound so tasty, there is a Djinni who will gladly take that cake off of your hands and grant you a “wish” in exchange – so long as that wish involves choosing one of the great prizes in his list including your choice of +2 ability tomes.

About the Djinni himself, as quoted from Cordovan :

The Djinni will be there to consume your cake whenever you are ready for him.

Apparingly this means that it really doesn't matter when you "summon" him.

Several people report the Djinni "stealing the cake", and giving nothing back in return. I fear that this might be a bug, so I'm going to wait for a few days now.

Edit : There are news about it :

Sixth Birthday Cake Known Issue
February 27, 2012
Having multiple players click on a Sixth Birthday Cake Djinni can cause issues with delivering the birthday items. Players should use their Djinni in a private area.

But 1 person in the forums said that this happened even on a Guild Ship - perhaps summoning within a quest works better ?

About the experience bonus which has begun yesterday, by Cordovan as well :

The 10% XP Bonus will be here through March 4th.

Also from the "open letter" :

In addition to the premium features we’ve already discussed, there will be a large number of technology upgrades, system updates and bug fixes which will improve the game experience for all of our players. For example, we are completely overhauling our enhancement system and filling out the PrEs for all classes! There will also be significant changes to help performance and reduce lag. We are releasing a lot of new information today, and we have plans for many more dev diaries and articles which will be posted in the coming months. Keep your eyes on DDO.com and the forums for more information.

For those of us who love to True Reincarnate, the new XP tomes will be especially handy and (and they thankfully persist through TR , as all tomes will now do with the launch of U13).

Another important thing we are announcing today is that Update 14 will be coming this August! Just two months after Menace of the Underdark goes live, we are planning to release our first premium Forgotten Realms adventure pack. It introduces an all new story line and our second Epic level raid in the game.

And last : A forums entry :

I just wanted to post to add that my account was also credited 2,000 TP when I applied the pre-order. (collector edition)

And besides : I think it is no wonder why the Fens are connected with the Expension's new theme … There are discussions among Drow within Stormreach …

-- “ Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius – and a lot of courage – to move in the opposite direction.“ (E.F.Schumacher, Economist, Source)